'◇ PhysicsⅠ◇': Search Results 33
  1. 2006/12/09 Solid-oxide fuel cell - From Wikipedia
  2. 2006/12/08 Solid Oxide Fuel Cells
  3. 2006/08/11 ‘고체로 수소 저장’ 물질구조 발견
  4. 2006/06/15 Numerov method
  5. 2006/04/12 생물학적 수소제조에 대하여
  6. 2006/03/03 [Cololr Centers]보석이 반짝이는 이유 (5)
  7. 2006/01/06 The Adiabatic Principle
  8. 2005/10/13 2005 노벨물리학상
  9. 2005/06/10 레일건
  10. 2005/05/29 거물들의 어록
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Solid-oxide fuel cell - From Wikipedia

Introduction

Solid oxide fuel cells, or SOFC, are intended mainly for stationary applications with an output from 1 kW to 2MW. They work at very high temperatures, typically between 700 and 1000ºC. Their off-gases can be used to fire a secondary gas turbine to improve electrical efficiency. Efficiency could reach as much as 70% in these hybrid systems, called Combined Heat and Power device (CHP). In these cells, oxygen ions are transferred through a solid oxide electrolyte material at high temperature to react with hydrogen on the anode side. Due to the high operating temperature of SOFC's, they have no need for expensive catalyst, which is the case of Proton-exchange fuel cells (platinum). This means that SOFC's do not get poisoned by carbon monoxide and this makes them highly fuel-flexible. Solid oxide fuel cells have so far been operated on methane, propane, butane, fermentation gas, gasified biomass and paint fumes. However, sulfur components present in the fuel must be removed before entering the cell, but this can easily be done by an active coal bed or a zinc absorbent.

Thermal expansion demands a uniform and slow heating process at startup. Typically, 8 hours or more are to be expected. Micro-tubular geometries promise much faster start up times, typically 13 minutes.[1]

Unlike most other types of fuel cells, SOFC's can have multiple geometries. The planar geometry is the typical sandwich type geometry employed by most types of fuel cells, where the electrolyte is sandwiched in between the electrodes. SOFC's can also be made in tubular geometries where either air or fuel is passed through the inside of the tube and the other gas is passed along the outside of the tube. The tubular design is advantageous because it is much easier to seal and separate the fuel from the air compared to the planar design. The performance of the planar design is currently better than the performance of the tubular design however, because the planar design has a lower resistance compared to the tubular design.

How a Solid Oxide Fuel Cell Works

Cross secton of the three ceramic layers of an SOFC. From left to right: porous cathode, dense electrolyte, porous anode
Cross secton of the three ceramic layers of an SOFC. From left to right: porous cathode, dense electrolyte, porous anode

A solid oxide fuel cell is made up of four layers, three of which are ceramics (hence the name). A single cell consisting of these four layers stacked together is typically only a few millimeters thick. Hundreds of these cells are then stacked together in series to form what most people refer to as a “solid oxide fuel cell.” The ceramics used in SOFCs do not become electrically and ionically active until they reach very high temperature and as a consequence the stacks have to run at temperatures ranging from 700 to 1200 °C.

Cathode

The ceramic cathode layer must be porous, so that it allows air flow through it and into the electrolyte. There are various types of ceramic materials used for the cathode, but all of them must be electrically conductive. The cathode is the negative side of the cell towards which electrons flow. It is the side that is exposed to air and its purpose is to use electrons to reduce the oxygen molecules in the air to oxygen ions.

Electrolyte

The electrolyte is the dense, gas-tight layer of each cell that acts as a membrane separating the air on the cathode side from the fuel on the anode side. There are many ceramic materials that are being studied for use as an electrolyte, but the most common are zirconium oxide based. Besides being air-tight, the electrolyte must also be electrically insulating so that the electrons resulting from the oxidation reaction on the anode side are forced to travel through an external circuit before reaching the cathode side. The most important requirement of the electrolyte however is that it must be able to conduct oxygen ions from the cathode to the anode. For this reason, the suitability of an electrolyte material is typically measured in ionic conductivity.

Anode

The ceramic anode layer must be very porous to allow the fuel to flow to the electrolyte. Like the cathode, it must conduct electricity. The most common material used is a cermet made up of nickel mixed with the ceramic material that is used for the electrolyte in that particular cell. The anode is commonly the thickest and strongest layer in each individual cell, and is often the layer that provides the mechanical support. Electrochemically speaking, the anode’s job is to use the oxygen ions that diffuse through the electrolyte to oxidize the fuel (hydrogen). The oxidation reaction between the oxygen ions and the hydrogen fuel produces both water and electricity.

Interconnect

The interconnect can be either a metallic or ceramic layer that sits between each individual cell. Its purpose is to connect each cell in series, so that the electricity each cell generates can be combined. Because the interconnect is exposed to both the oxidizing and reducing side of the cell at high temperatures, it must be extremely stable. For this reason, ceramics have been more successful in the long term than metals as interconnect materials. However, these ceramic interconnect materials are extremely expensive. Fortunately, inexpensive metallic materials are becoming more promising as lower temperature (600-800°C) SOFCs are developed.

Research

Research is going now in the direction of lower-temperature SOFC (600ºC) in order to decrease the materials cost, which will enable the use of metallic materials with better mechanical properties and thermal conductivity.

Research is also going on in reducing start-up time to be able to implement SOFC's in mobile applications. Due to their fuel flexibility they may run on partially reformed diesel, and this makes SOFC's interesting as auxiliary power units (APU) in refrigerated trucks.

Specifically, Delphi Automotive Systems and BMW are developing an SOFC that will power auxiliary units in automobiles. A high-temperature SOFC will generate all of the needed electricity to allow the engine to be smaller and more efficient. The SOFC would run on the same gasoline or diesel as the engine and would keep the air conditioning unit and other necessary electrical systems running while the engine shuts off when not needed (e.g., at a stop light).

Rolls-Royce are developing Solid-Oxide Fuel Cells produced by screen printing onto inexpensive ceramic materials. Rolls-Royce Fuel Cell Systems Ltd is developing a SOFC gas turbine hybrid system fuelled by natural gas for power generation applications generating power of the order of a megawatt.[2]

Ceres Power Ltd. are developing a low cost and low temperature (500-600 degrees) SOFC using cerium gadolinium oxide in place of current industry standard ceramic (yttria stablised zirconia) which allows the use of stainless steel to support the ceramic.

See also

Notes and references

  1. ↑ Sharke, Paul (2004). "Freedom of Choice". Mechanical Engineering 126 (10): 33.
  2. ↑ Adamson, F (2004). "Propagating Reaction Fronts in Zirconia Tubes". PhD thesis.

P. Batfalsky, V.A.C. Haanappel, J. Malzbender, N.H. Menzler, V. Shemet, I.C. Vinke, R.W. Steinbrech, Chemical interaction between glass–ceramic sealants and interconnect steels in SOFC stacks, Journal of Power Sources, 155 (2006) 128.

J. Malzbender, T. Wakui, R.W. Steinbrech, L. Singheiser, Deflection of Planar Solid Oxide Fuel Cells During Sealing and Cooling of Stacks, Fuel Cell 2 (2006) 123.

External links

2006/12/09 17:04 2006/12/09 17:04
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Solid Oxide Fuel Cells

출처 : www.corrosion-doctors.org/FuelCell/sofc.htm



Solid Oxide Fuel Cells

SOFCs have recently emerged as a serious high temperature fuel cell technology. Of primary importance is the fact that SOFCs require no liquid electrolyte, with associated Corrosionand electrolyte management problems. This system is based upon the use of a solid ceramic as the electrolyte and operates at extremely high temperatures (1000°C). This high operating temperature allows internal reforming, promotes rapid electrocatalysis with non-precious metals, and produces high quality byproduct heat for cogeneration. It is best suited for provision of power in utility applications due to the significant time required to reach operating temperatures. Programs are underway in Japan and in U.S. The development of suitable materials and the fabrication of ceramic structures are presently the key technical challenges facing SOFCs. A schematic description of the components in a SOFC is shown here:

Solid oxide fuel cell

The electrolyte typically consists of a solid non-porous such as Y2O3 stabilized ZrO2 with conductivity based on oxygen ions (O2-). Typically the anode is made of a Co-ZrO2 or Ni-ZrO2 cermet, and the cathode of Sr doped LaMnO3. The solid state character of all SOFC components means that there is no fundamental restriction on the cell configuration. Cells are being constructed in two main configurations, i.e. tubular cells, such as those being developed at Westinghouse Electric Corporation since the late 1950s, and a flat plate configuration adopted more recently by many other developers.

EPRI considers SOFCs, which employ a ceramic, solid-state electrolyte (zirconium oxide stabilized with yttrium oxide), the only fuel cell technology with the potential to span market-competitive applications from residential loads as small as 2 kW to wholesale distributed generation units of 10 - 25 MW. Because SOFCs operate at a higher temperature than MCFCs, their simple system efficiency is theoretically not quite as good as that of MCFCs, although it is better than the efficiencies of PAFCs and PEM fuel cells. But the 850 - 1000°C waste heat that SOFCs produce, when used for cogeneration or for driving an integrated gas turbine, can boost overall system energy efficiency to very attractive levels. Moreover, SOFCs operate at a high enough temperature to incorporate in their an internal fuel reformer that uses heat from the fuel cell, along with recycled steam and a catalyst, to convert natural gas directly into a hydrogen-rich fuel.

High-efficiency systems coupling advanced SOFCs with small gas turbines and having a combined rating in the range of 250 kW to 25 MW are expected to fit into grid-support or industrial on-site generation markets, and they potentially could compete head-on with wholesale power rates. Both PEM fuel cells and SOFCs could someday be suitable for small-scale residential market applications if ultimate cost goals are reached, i.e. $1000/kW.

2006/12/08 14:30 2006/12/08 14:30
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‘고체로 수소 저장’ 물질구조 발견
‘수소車상용화’ 한발짝 다가섰다

2006년 08월 05일| 글 | 김동욱 동아일보 기자ㆍcreating@donga.com |

수소자동차 상용화를 앞당길 수 있는 새로운 물질 구조가 국내 연구팀에 의해 발견됐다. 서울대 물리천문학부 임지순 교수 연구팀은 수소를 고체 상태에서 저장할 수 있는 물질 구조를 발견했다고 4일 밝혔다.

이 연구결과는 물리학 분야에서 세계 최고 권위의 학술지인 ‘피지컬 리뷰 레터’에 이날 게재됐다.

연구팀은 슈퍼컴퓨터를 이용해 수백 가지의 다양한 물질 구조를 설계하는 과정에서 플라스틱을 이루는 물질인 ‘폴리머’를 뭉치지 않게 분산시켜 ‘티타늄’ 원자를 달면 다양한 수소가 달라붙어 안전하게 저장된다는 사실을 발견했다.

연구팀은 “이런 구조로 상온·상압에서 수소를 저장하면 그 저장량이 2010년 미국 에너지부 목표치보다 25% 이상 초과할 정도로 획기적으로 늘어나게 된다”고 밝혔다.

수소자동차는 세계적인 자동차 회사들이 경쟁적으로 개발을 시도하고 있으나 수소를 가스 상태로 탱크에 저장하면 부피가 크고 폭발 위험을 안고 있어 안전한 저장물질을 찾는 것이 학계와 산업계의 오랜 숙제였다.

이번에 발견된 물질 구조를 수소자동차 상용화 개발에 응용하면 이 과제를 해결할 수 있다.







서울대 물리천문학부 임지순 교수 연구팀이 발견한 수소를 고체 상태에서 저장할 수 있는 물질 구조. a)는 폴리아세틸렌에 티타늄이 결합된 상태에 수소(빨간색)가 붙어있는 모습. b)는 폴리아닐린에 티타늄이 결합된 상태에 수소들이 붙어있는 모습. 자료 제공 서울대

임 교수는 “이번 연구로 청정에너지와 대체에너지로 관심을 받고 있는 수소 에너지 개발 분야의 경쟁에서 유리한 위치를 확보하는

한편 세계적으로 경쟁이 치열한 수소자동차의 상용화에서도 한발 먼저 앞으로 나아갔음을 증명했다”고 설명했다.


연료전지에 관련해서 좋은 논문이 나왔나보네;;
내가 하는 연구도 돈벌이가 되는 결과가 나와야 할텐데 말야 ㅋ
2006/08/11 18:14 2006/08/11 18:14
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Numerov method
2006/06/15 15:30 2006/06/15 15:30
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생물학적 수소제조에 대하여

출처 :  http://www.h2.re.kr/


   태양광을 에너지로 이용하며, 물이나 유기물질로부터 미생물 내에 존재하는 자가 증식형 메카니즘에 의한 수소 생산은 에너지 생산 기술일 뿐만 아니라, 공기중 이산화탄소를 변환하여 탄수화물로 미생물 내에 축적하는 이산화탄소 저감 환경기술이며, 유기물질 농도가 높은 식품계 공장 폐수에 적용할 경우 유기물질로부터 수소를 생산하고 폐수의 COD를 낮추는 환경처리도 할 수 있다. 또한 광합성 미생물 자체에 축적되는 β-carotene, astaxanthin과 같은 고부가가치 식․의약품의 생산기술로도 활용되고 있다.
   생물학적 수소생산 기술은 다양하여 기질로 사용되는 원료물질에 따라 물, 유기물, 가스로 크게 구분되며, 또한 미생물의 다양한 메카니즘에 따라 여러가지 기술이 알려져있으며 아직도 새운 기술 및 다양한 수소 생산 미생물에 대한 연구가 왕성하다. 이 중에서도 ① 녹조류 (green algae)가 광합성 메카니즘에 의해 물로부터 양성자와 전자를 공급받아 수소를 생산하는 직접 물 분해 수소생산 기술(direct bio-photolysis) ② 광합성 작용에 의해 물을 분해하여 산소를 발생하고, 동시에 공기 중 이산화탄소를 고정하여 고분자 저장물질로 균체 내에 합성한 후 혐기 발효 또는 광합성 발효에 의해 수소를 발생하는 간접 물 분해 수소생산 기술(indirect bio-photolysis or two stage photolysis) ③ 최근 일본을 비롯한 유기성 폐자원이 풍부한 국가에서 집중적으로 연구되는 기술로써, 유기물로부터 purple non-sulfur bacteria에 의한 광합성 발효에 의한 수소생산(photo-fermentation) 또는 ④ 광이 존재하지 않는 조건에서 혐기 미생물에 의해 유기물 자체가 에너지원으로  사용되는 발효에 의한 수소생산 기술 (dark fermentation) ⑤ 광합성에 관여하는 엽록체 및 미생물 효소를 추출하여, 물 또는 유기물로 부터 수소를 발생하는 균체 외 (in vitro) 수소 발생 ⑥ 광합성 미생물의 일산화탄소 가스 전환 반응 (microbial shift reaction)에 의한 수소 생산 기술로 구분할 수있다. 

   국내의 산업 및 자연환경에 가장 적합하고 실용화 가능성이 높은 것은 무엇일까?

   첫 째 유기물로부터 혐기미생물을 이용한 dark-fermentation과 광합성 미생물을 이용한 photo-fermentation을 연속적으로 적용하여 유기성 폐수 및 폐기물로부터 최대 효율로 수소를 생산하는 연구와, 둘째 미생물이 생산하는 수소생산 관련 효소 및 광합성 시스템을 균체 외로 분리하여 물 또는 유기물로부터 in vitro 방법에 의한 수소생산을 개발하여, 수소에너지 시스템과 연계한 대량 생산이다.  

   혐기 및 광합성 발효에 의한 유기물질로부터 수소생산은 최근 국내 및 일본을 비롯한 유기성 폐자원이 풍부한 나라에서 집중적으로 연구되는 기술로서, 유기물로부터 혐기 및 광합성 미생물을 각각 적용시킴으로써 수소를 생산하는 즉, 유기물 자체가 에너지원으로 사용되는 발효에 의한 수소발생 기술이다.
   수소를 생산할 수 있는 세균 중에는 빛이 없는 혐기 발효 조건에서 유기물을 이용하여 배양액 중에 각종 유기산, 유기용매를 축적하고, 동시에 수소와 이산화탄소를 발생한다.  클로스트리디움 속은 가장 잘 알려진 혐기 발효 수소생성 세균이며 현재 이들을 이용한 수소생산에 관한 연구가 활발히 진행되고 있다.


       C6H12O6(glucose) +6H2O → 2CH3COOH (acetic acid) + 4H2 + 2CO2

       C6H12O6(glucose)  → 2CH3(CH)2COOH (butyric acid) + 2H2 + 2CO2


   위의 경우 glucose 1 분자는 혐기 미생물이 갖는 자체 내 발효 메카니즘에 의해 2 분자의 acetic acid와 동시에 4분자의 수소를 생산한다. 생성되는 수소양은 어떠한 유기산이 생성되는가에 따라 차이는 있지만, butyric acid가 생성될 경우는 2분자의 수소가 발생한다.

   이와 같은 수소생성량은 glucose 1분자로부터 최대 생성되는 12분자 수소 중 4분자만이 생성되므로 약 33% 전환에 불과하지만, 동시에 발생하는 유기산 즉 acetic acid나 butyric acid 등은 광합성 세균에 의한 발효로 다시 수소 생산을 유도할 수 있다. 즉,


       2CH3COOH (acetic acid) +4H2O → 4CO2 + 8H2


   광합성 세균은 조류나 식물이 PS I(Photosynthetic system I) 과 PS II (Photosynthetic system II)를 모두 광합성에 이용하는 것과는 달리 PS I 만을 이용하여 광합성과 수소생산을 한다. 즉 cytochrome 색소 복합체로 구성된 반응계(reaction center)가 있어서 빛에너지를 색소가 흡수하면 반응계의 전위차가 형성되어 cyclic 전자 전달계를 생성하며, 이때 ATP라는 고 에너지 화합물을 합성한다. 한편 기질로부터 공급된 전자는 ferredoxin에 전달되고, 이 환원력과 ATP를 이용하여 nitrogenase 효소는 양성자(H+)를 수소 H2를 환원한다. 광합성 세균은 대사적인 다양성을 갖고있어 산소가 있을 경우나 없을 경우 모두 성장 할 수 있고 , 광합성 작용으로 수소를 생산할 수 있다. 이러한 다양성 때문에 기질의 이용효율에 차이는 있지만 단당류, 이당류 및 각종 유기산을 모두 배양 기질로 사용할 수 있어서 실질적으로 수소 생산을 쉽게 유도할 수 있다. 광합성 세균 중에서 대표적으로 이용되는 홍색 비유황 세균(purple non-sulfur bacteria)는 이론적으로 acetic, lactic, 또는 butyric acids로부터 각각 4, 6, 7분자의 수소가 생성된다. 정리하면 glucose 1분자로부터 혐기 세균과 광합성 세균을 적용할 때 최대 12분자의 수소가 발생하지만 실질적으로는 미생물의 배양조건 즉 pH 변화, 빛 이용효율, 온도등에 의해 최대 8-9분자가 발생하는 것으로 보고되고 있다. 유기물질이 다량 함유되어 있는 식품계 공장폐수나 하천 슬러지, 농수산 시장의 폐기물은 이와 같은 혐기 및 광합성 세균을 이용하여 수소를 생산할 수 있는 좋은 바이오매스로써 에너지 생산과 환경 처리를 동시에 할 수 있는 것으로 타당성이 검토되어 국내외에서 기술이 개발 중이다.
2006/04/12 15:47 2006/04/12 15:47
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[Cololr Centers]보석이 반짝이는 이유

녹색 다이아 : missing carbon atom which absorbs red light


Amethyst is a violet tinted gemstone consisting of crystalline quartz with iron as an added impurity. The iron impurity in amethyst acts differently from the chromium impurity in ruby which makes ruby red. In ruby the impurity is excited directly by the "visible" photon; in amethyst, it is not. Instead, the impurity in amethyst creates another unstable species in the crystal and this new species jumps to an excited state by absorbing a "visible" photon. This species is known as a color center. By absorbing a "visible" photon, it makes the crystal appear colored.

The simplest color center is found in sodium chloride or table salt, normally a colorless crystal. Sodium chloride is built from little cubes, with sodium ions Na+and chloride ions Cl- alternating at each corner. Every Na+ is surrounded by Cl- and vice versa; and the attraction between the positive and negative charges makes salt a strong crystal. When salt is bombarded by high energy radiation (cosmic rays from outer space or gamma rays from a radioactive source), a Cl- can be ejected from the crystal, creating a vacancy where the Cl- had been. Momentarily the crystal is no longer electrically neutral: it is missing a negative charge. To regain stability, it grabs an available electron close to the crystal and sticks it in the vacancy previously occupied by the ejected Cl-. The crystal then appears as in the following figure.

Figure 10.7, Tilley, p.240

With the electron replacing the ejected Cl-, there are now equal numbers of positive and negative charges in the crystal; and the electron is held firmly in its site by the surrounding positively charged NaNow the crystal has an electron in place of the original Cl-+ ions. This process turns the colorless salt crystal into an orange/brown. The trapped electron, responsible for the color, is known as a "color center." The color center produces the color exactly as the impurity does in the ruby crystal. The color center can also exist in an excited state, and the energy needed to reach that excited state is equal to the energy of a visible photon. The color center actually absorbs a "violet" photon by jumping to the excited state; and the crystal appears with the color orange/brown (the complement of violet).g

Analogous color centers occur in several minerals. In some cases, impurities are involved in forming the color centers.

An iron impurity is responsible for the violet color in amethyst by creatiing a color center. The color center, not the iron impurity, is responsible for absorbing the "yellow" photon that makes amethyst violet. In another example, topaz, a fluoroaluminosilicate with iron as an added impurity, achieves its yellow color by means of a color center, again created by the iron impurity.




Summary

Why are some crystals colorless and others colored?

  • Colorless crystals are pure crystals such as diamonds, quartz and corundum. The atoms in a pure crystal form a rigid, regular framework. Neighboring atoms in the framework bind to each other by sharing pairs of electrons in strong chemical bonds. Pure crystals form states of higher energy by "uncoupling" the electron pairs. That takes more energy than is available in a "visible" photon. They are unable to absorb "visible" photons, so they appear colorless.
  • Colored crystals are pure crystals, made impure by adding impurities. The impurities reduce the energy needed to reach higher-energy states. This energy is now comparable to the energy of a "visible" photon. The crystal will absorb a "visible" photon and appear colored.

In some cases (e.g. ruby), an impurity will achieve this if the impurity has an excited state of the right energy. In other cases (e.g. sapphire), an excited state of the right energy can be achieved by charge transfer (or, transfering a charge between) between two impurities. Again (e.g. in amethyst) a new species, a color center, with an excited state of the right energy, can be created within the crystal, formed using vacancies and, often, impurities.

(This page is under development, and might be completed Winter 2002).

Take a century-old glass bottle, and expose it in the desert to the ultraviolet radiation present in strong sunlight. Come back after ten years, and the glass will have acquired an attractive purple color. Heat the bottle in an oven, and the color disappears. Next expose the bottle to an intense source of energetic radiation, as in the cobalt-60 gamma ray cell of Figure 24, and within a few minutes an even deeper purple color appears, as shown in Plate XI.

The color in this "desert amethyst glass" derives from a color center, as do the colors of the natural gemstones amethyst, smoky quartz, and blue and orange topaz. Many other materials, both natural and man-made, can be irradiated to produce color centers, including irradiated blue, yellow, and green diamonds. Some of these colors, such as all the ones mentioned so far, are perfectly stable, losing their color only when heated. Other color centers are unstable and fade when exposed to light, while yet others fade even in the dark.

The term "color center" is sometimes used so loosely that even transition-metal and the band-gap colorations are included. This rare usage ignores the unique characteristics of color centers; the conventional narrow interpretation is followed here.

Consider an ionic crystal, such as the alkali halide sodium chloride NaCl (ordinary table salt), which consists of a three dimensional array of Na+ and Cl- ions. A single Cl- can be missing in two ways. If a compensating Na+ is also missing, then the crystal remains neutral and there are no consequences of interest with respect to color. If, however, a Na+ is not missing, then one way of maintaining electrical neutrality is for a free electron, designated c, to occupy the spot vacated by the Cl-. This is called an F-center, after the German "Farbe" (color), as shown at the top left of Fig. 25. One can view this electron as if it were part of a transition metal in the ligand field of the surrounding K' ions or, preferably, one can view this electron as providing a trapping energy level within the band gap of this transparent wide-band-gap semiconductor material, as shown in Fig. 26.

Some form of relatively high energy such as irradiation by ultraviolet or high-energy electrons, x-rays, or gamma rays can now promote an electron from the valence band into the trap. There are, however, excited energy levels within the trap, such as the level at Ea (at 2.7 eV for NaCI), which can absorb blue light, leading to a yellow-brown color in irradiated defect- containing NaCl; this defect is now called a color center. Note that the electron in this excited energy level is still within the trap. Only by supplying energy corresponding to Eb can the electron leave the trap and return via the conduction band directly to the valence band. This can happen if the crystal is heated, and results in bleaching of the color center. If Eb is about the same size as Eb, then bleaching can occur merely while the material is being illuminated, leading to optical bleaching. If Eb is sufficiently small, the material may even fade in the dark at room temperature. This occurs in self-darkening sun glasses, in which the ultraviolet present in sunlight produces the darkening and room temperature leads to fading as soon as there is no ultraviolet. Other centers are possible in alkali halides, some of which are also shown in Fig. 25; these may absorb in the visible, the ultraviolet, or the infrared. Some such color centers also show fluorescence and some of these can function as laser materials. As alternatives to irradiation, growth in the presence of excess metal or solid-state electrolysis can also be used to generate color centers.

The most general description of a material capable of supporting a color center is given in Fig. 27, in which the colorless state is shown above and the colored state below. Two kinds of precursors are needed: a hole precursor A which can lose an electron, e.g., when absorbing irradiation, to form a hole center A+, and an electron precursor B which can gain the electron lost from A to form the electron center B-. Either A+ or B- can be the color center itself that absorbs light, or even both can do so. On heating, the electron is released from B- and returns to A+', thus restoring the colorless state of A plus B.

A number of gemstone materials derive their beauty from color centers. Colorless "rock-crystal" quartz, shown center above in Plate XI, is composed of silicon oxide SiO2, shown schematically at A in Fig. 27. All natural and synthetic quartz contains the aluminum impurity Al3+, typically replacing one out of every 10,000 Si"; for charge neutrality a hydrogen ion H+ or a Na+ is nearby. Such quartz is colorless, but irradiation, either natural in the ground over many thousands of years or man-provided in 20 minutes in a cobalt-60 gamma source such as that of Fig. 24, now produces smoky quartz, also shown in Plate X1. As illustrated at B in Fig. 28, irradiation ejects an electron from an oxygen adjacent to the Al3+, the whole [AlO4] grouping acting as the hole precursor and converting to the hole center [AlO4]. The electron is trapped by the H+ electron precursor, converting it into the neutral H electron center. In this case it is the hole center that is the color center and provides the gray-to-brown-to-black color of smoky quartz seen in Plate XI. Also shown in this figure is yellow citrine (often erroneously called "smoky topaz"), which is quartz containing Fe3+ instead of Al3+; this produces the purple amethyst, also shown in Plate XI, by an exactly analogous irradiation process leading to the hole color center [FeO4].

The colors of both amethyst and smoky quartz are stable to light but are lost on being heated to 300 to 500"C; if not overheated, the color center and the color can be restored by another irradiation, and so on.

A century ago, glass used to be decolorized with manganese additions to remove the green color caused by iron impurities. It is the Mn2+ left from this process which loses an electron to form the purple Mn7+ shown in Plate XI in the solarization process described at the beginning of this section.

Natural yellow-to-orange-to-brown precious topaz contains a color center stable to light; any colorless topaz can be irradiated to a similar color that, however, is usually unstable and fades in a few days in light. Blue topaz also contains a color center, which can be either natural or manproduced; here both are stable. The exact nature of most of these color centers is unknown. Interestingly enough, the irradiation of colorless diamonds can produce stable yellow, blue, brown, and green colors. Although the first two of these are similar in appearance to the N-caused yellow and the B-caused blue discussed above, they represent much less valued materials, which can be distinguished by spectroscopic and other features.

FIG. 24. A sample being placed into a gamma-ray cell for irradiation by the author.

PLATE XI. Color centers. Above: century-old glass bottle irradiated to form "desert amethyst glass," colorless syntheticquartz crystal as grown, and one that has been irradiated to form smoky quartz. Below: a synthetic citrine quartz colored yellow by Fe and one that has been additionally irradiated to form amethyst.

FIG. 25. Different types of color-center defects in an ionic crystal (schematic).

FIG.26. Trapping of energy from absorbed light in a halide vacancy trap in an alkali-halide crystal.

FIG. 27. The irradiation of hole and electron precursors (a) to form hole and electron centers (b).

FIG. 28. Schematic representation of the structure of quartz (A) and the formation by irradiation of a smoky- quartz color center (B).


2006/03/03 21:16 2006/03/03 21:16
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The Adiabatic Principle


The Adiabatic Principle


There is a fundamental principle in physics, found in slightly different forms in mechanics, thermodynamics, and quantum theory, and generally known as the adiabatic principle. Its basic use in physics is to simplify complex analyses by justifying the neglect of certain possible (but hard to calculate) interactions as being almost certainly too small to make a noticeable difference in the final answer (the adiabatic approximation). ‘Adiabatic’ basically means ‘it doesn’t get through’ referring to energy, fields, or information. In its most basic form it is a statement about energy transfer, and it says that it takes time for energy to be transferred from one system to another; therefore the faster something happens, the less energy is transferred. This means, in effect, that a very fast and a relatively much slower process cannot efficiently communicate with one another, cannot transfer energy. This is the basic warrant for the buffering or filtering effect between non-adjacent levels in the timescale hierarchy, and therefore for the usefulness of defining timescales as being distinct from one another in the first place. 

A process which produces change only very slowly seems to us not to be a process at all, but a constant fact of life. Very slow changes do not produce ‘differences that make a difference’ (Bateson 1972) to us; they do not matter to human life. Weather change processes make a big difference to us, but climate change processes are so slow as to be irrelevant (normally, but that may be changing!). The continents are moving, the Earth’s magnetic poles are shifting, the equinoxes are precessing, the rotation of the earth is slowing, the energy output of the sun is changing -- but not fast enough to matter to our sense of geography or day and night. 

Or consider very fast processes, much faster than those at our nominal one-second focal level. If you run fast enough across the hot beachsand your feet get less burned because less total energy is transferred to you in the shorter time (for hot coals you may need additional help.) The extreme case was graphically illustrated in a recent film of H.G. Wells’ classic The Time Machine, in which the protagonist survives a nuclear blast in London by accelerating through time at the maximum rate, thus spending too little time in the actual moments of blast energy for very much of it to transfer to him and the machine. Closer to home, fast molecular and atomic processes within the human body do not play a role in our much slower biochemistry, nor can we decipher speech presented to us more rapidly than the maximum rate at which our neurons can respond and process the signals. Moreover, and this goes beyond and adds to the separability of timescales guaranteed by the adiabatic principle, we are buffered from fast, small-scale events, like ionization of individual atoms in our bodies or even errors in gene transcription, by longer term regulatory and self-correcting processes typical of the intermediate scales of autopoietic or self-organizing systems. 

Of course our small degree of autonomy from the environment, within and without, at smaller scales and larger ones, has its distinct limits. One molecular error in one cell can sometimes lead to a cancer that kills the organism. Someday we may cross a threshold in long-term climate change processes and find sudden droughts and famines on a very human timescale. The adiabatic principle has exceptions, and one of these is fundamental to human social organization.

출처 : http://www-personal.umich.edu/~jaylemke/webs/time/mca-adiabatic.htm
2006/01/06 22:15 2006/01/06 22:15
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2005 노벨물리학상

 

2005년 노벨물리학상-양자광학분야 3명



노벨 물리학상은 양자광학 이론을 확립한 로이 글라우버(80) 미국 하버드대 교수와 초정밀시계·위치확인시스템(GPS) 등에 쓰이는 레이저 정밀 분광학을 개발한 존 홀(71) 미국 표준연구원(NIST) 선임연구원과 테어도어 헨슈(64) 독일 루트비히-막시밀리안대 교수 등 3명에게 돌아갔다.

스웨덴 왕립과학원은 4일(한국시각) “올해 물리학상은 광학 분야에서 이론적·기술적 성과를 거둔 과학자 3명이 받았다”며 “노벨상의 반은 양자 이론을 광학에까지 확장하는 양자광학 이론의 기초를 확립한 글라우버에게, 나머지 반은 레이저 기반 정밀 분광기술을 개발한 홀과 헨슈에게 주어졌다”고 밝혔다.

글라우버는 1850년대 맥스웰의 전자기파 이론과 1905년 아인슈타인의 광전효과 이론을 토대로 양자광학 이론을 정립했다. 맥스웰은 전기현상과 자기현상이 다르지 않다는 이론을 제기해 전자기파의 존재를 예측했다. 이는 마르코니가 무선전신을 개발하는 이론적 기틀이 됐다. 아인슈타인은 더 나아가 전자기파, 곧 빛이 파동성뿐만 아니라 입자성을 지니고 있다는 광전자이론을 세웠다. 그는 빛의 알갱이인 광자가 금속과 부딪칠 때 튀어나오는 전자로 빛의 입자성을 설명했으며, 이 이론으로 1921년 노벨상을 받았다.

글라우버는 백열등 등 금속이 달궈질 때 나오는 빛은 전자들이 제멋대로 튀어나오는 현상인 데 비해 레이저의 빛은 주파수와 위상(파동에서의 위치)이 같은 ‘양자광학적 결맞음’ 상태에 있음을 규명했다. 말하자면 백열등 빛은 거리에 사람들이 걸어가는 모습이라면, 레이저는 줄을 맞춰 행진하는 것에 비유할 수 있다. 글라우버는 1963년 미국 물리학회에서 발행하는 <피지컬 리뷰 레터스>에 ‘광자상관관계’라는 논문을 통해 이 이론을 처음 제기했으며, 이 결맞음 현상을 수식화함으로써 아인슈타인과 함께 양자광학의 중요한 창시자가 됐다.

홀과 헨슈는 빛을 머리빗처럼 가늘게 쪼개는 기술을 개발해 10의 마이너스 15승분의 1 정도의 정밀도를 지닌 분광기기 제작의 토대를 마련했다. 김재완 고등과학원 교수(양자정보)는 “원자 등 물질의 내부 구조를 알기 위해서는 전자의 준위(높낮이)가 바뀌면서 나오는 빛을 측정해야 할 정도로 정밀한 도구가 필요하다”며 “이런 측면에서 홀과 헨슈의 공로가 인정된 것 같다”고 말했다.

한계레 신문보도.

 


2005/10/13 20:49 2005/10/13 20:49
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레일건






퀘이크를 하면 레일건이 나온다.

퀘이크를 먹고서 한방쏘면 원샷원킬이란 무시무시한 무기

하지만 가까운 미래에는 존재 할 법한 무기였군.

아래 레일건의 간단한 제작원리에 관한 글이 있기에 퍼왔다


네이버 지식인에 올라온 pantorm님이 작성한 글

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현존하는 개인화기 중에서 두께 1m 콘크리트를 관통하는 총은 없습니다.
여러명이 운용하는 포와 같은 무기라면 모를까...

있다고 한다면....
아마도 개인용 대전차무기를 말하는 것 같습니다.
예를 들어 판저 파우스트3나, RPG-7이나...

앞으로 그런 무기가 나온다는 말은 많습니다.
그 중에서 가장 유력한 무기는 '레일 건'입니다.

레일 건의 원리는 기다란 두개의 특수금속 막대(레일) 사이에 전기를 가해서
레일 건의 탄환인 특수 알루미늄 탄환이 발사되도록 하는 겁니다.

그렇게 발사된 탄환은 두께 50cm 이상의 금속판 20장 이상을
관통할수 있다고 합니다.

그것이 실용화 된다면,
레일건에 정밀 조준기를 달아서 적 전차의 연료탱크를 쏘는 것이 가능해 집니다.
그럼 보병이 전차 잡는것은 껌이 되겠죠...

그러나 현재의 기술로 이것은 불가능 합니다.

탄환자체의 제작도 문제이지만,
탄환을 발사시킬만한 자기력의 발생과
발사대 역할을 하는 금속의 미묘한 성분조정이 너무나 어렵다고 하는군요.

그리고 설사 만든다고는 해도,
특수알루미늄탄을 극고의 속도로 발사시킬 전기력을 얻는데엔
엄청난 에너지가 소모됩니다.
현재의 기술로 이 발사 에너지 1회분을 마련하는 데에,
수소 밧데리 3트럭이 필요하다고 합니다...

즉, 만들더라도 개인용으로 만드는 데엔 무리라 이거죠...

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그 이외에 여기에도 관련된 글이 자세히 올라와 있어서 좀더 자세한 사항은 아래로

http://blog.naver.com/jjahn71/6861477



2005/06/10 19:55